The present invention relates to a method of recording fingerprints of a person""s finger area and more particularly to a method of providing a fingerprint on a recording surface containing a developing substance which reacts with an inkless reagent solution to form a colorant product representative of the ridge pattern of a person""s fingerprint area. It is to be noted that, as used herein, the term xe2x80x9cfingerprintxe2x80x9d encompasses prints of an individual""s hands or fingers or feet, such as those taken from newly born infants.
Fingerprints have become a universal method of identifying individuals. Fingerprint identification is an exacting science since two impressions of even the same fingerprint can appear different due to variations in the amount of chemical, such as ink deposited onto the recording surface, chemical migration and changes in the finger itself. To determine an exact correspondence a trained fingerprint technician or an automated machine reader compares the pattern of ridge endings and ridge bifurcations (minutiae) which are invariant with time on each person""s fingerprint.
The prior art has recognized that any viable fingerprint identification system requires a clear distinct print pattern with a minimum of chemical migration between adjacent ridges. An additional requirement for any voluntary print identification system, such as to be utilized commercially with checks, credit verification, verification of the recipient of a package or document, and the like, is that it be inoffensive to the person whose fingerprint is being obtained. Carbon based ink fingerprinting systems are particularly offensive because the ink stains the finger and must be removed. Furthermore, such systems tend to result in a smudging of the prints when the fingerprint documents, such as checks, etc, are handled. Inkless fingerprinting systems have been developed to overcome the above disadvantages of carbon based ink systems.
See, for example, U.S. Pat. Nos. 4,182,261 and 4,262,623 assigned to the assignee of this application and the references cited therein. Inkless systems generally rely on a chemical reaction between an invisible reagent deposited onto a porous recording surface such as paper or a card in the form of a latent fingerprint image and a developer which is applied to the surface before or after the application of the reagent. The reagent and developer react chemically to form a colored pattern, on the recording surface, representative of the ridge pattern of a person""s fingerprint. A large number of suitable reagent developer pairs for this type of application are disclosed in U.S. Pat. Nos. 4,029,012 and 4,182,261. It has also been suggested in U.S. Pat. No. 2,235,632 that the latent invisible fingerprint deposited from the person""s finger on the recording surface may be developed by a colorless powder, or volatile vapors comprising the developer or by the application of heat or intense light.
The application of a developer to the fingerprint paper or card just before or after the finger has deposited the reagent to provide the chemical reaction necessary to transform the invisible latent image into a visible one is time consuming and requires that an additional chemical be applied to the recording surface as compared with carbonized ink systems. The application of heat or intense light suffers from the obvious disadvantage that additional appendages to the fingerprinting apparatus, such as a heat or light source, may be required. Also, the operator of the fingerprinting apparatus cannot immediately judge whether or not the fingerprint taken is acceptable, since he or she has to wait until the print is developed by the application of heat or light.
U.S. Pat. No. 4,983,415 incorporated herein by reference, discloses an inkless system in which an acidic metal, e.g., a ferric salt solution, is used to develop the color former inherent in ordinary thermal paper. While the ""415 system provides an excellent permanent print, the ferric chloride solution is very acidic and therefor corrosive to metals. The system, depending upon the metallic salt used, may also react slowly with common thermal papers and provide an off black image, e.g., greenish-gray. Such metallic salt solutions are also fairly expensive.
There is a need for an inkless fingerprint method which relies on thermosensitive paper to develop the image and utilizes relatively inexpensive reagents to react with the paper.
In accordance with the present method a thermosensitive recording surface is provided which contains a thermosensitive color developing layer, the layer comprising chromogenic dyes, an organic acid developer reactive with the dye to develop a color and thermosensitive barrier for separating the acid from the dye. A solution of a monohydrogen or dihydrogen ester having an AAV value of about 9 or more is applied to the fingerprint area of the person to be printed and the fingerprint area is then pressed onto the recording surface whereby the solution reacts with the dye to provide a colorant product representative of the pattern of ridges and ridge endings of the fingerprint area.